外核海表熱通量對於梅姬颱風(2016)大小的影響

Abstract

颱風雨帶的位置以及活躍程度會影響颱風的大小(size)，並且過去的研究指出雨帶產生的次環流會影響颱風底層的角動量平流，而底層的角動量平流越多越有利於颱風切向風風場的擴張。除此之外，移除海表熱通量會減弱颱風雨帶的活躍程度。因此，本研究將探討海表熱通量以及颱風大小之間的關係，並瞭解海表熱通量如何影響颱風大小的改變。 本研究使用Advanced Research Weather Research and Forecasting (ARW-WRF) model (version 3.5.1)，以2016年的梅姬颱風(Megi)做為模擬對象進行模擬，並分為控制組實驗以及敏感性實驗。在敏感性實驗中，海表熱通量在不同區域被限制，限制區域分別為限制內核、外核以及整個第三層網格，並且限制程度也有所差異。研究顯示海表熱通量越多，颱風的大小越大，並且外核的海表熱通量對於颱風大小變化較為敏感。減少海表熱通量將導致該區域的對流活動減弱，並使得次環流也有所減弱。然而，由減弱的內雨帶引發的次環流減弱只侷限在內核區域，因此對於外核底層的角動量平流沒有太大的影響；另一方面，較不活躍的外圍雨帶引起的次環流減弱在徑向方向上較前述實驗寬，因此外圍底層的角動量較少，並且不利於颱風切向風風場的擴張。因此，海表熱通量越多越能讓颱風大小增大，並且外核的海表熱通量相對於內核的海表熱通量對於颱風的大小變化較為敏感。

Locations of outer rainbands and the embedded convective activities can affect tropical cyclone (TC) size, and it has been suggested that the secondary circulation induced by the rainbands can affect the momentum transport in the lower level, and more momentum import in the lower level leads to the expansion of TC. In addition, removing surface heat fluxes weakens the rainband activities. Therefore, this study attempts to further understand the impact of surface heat fluxes on TC size and how do surface heat fluxes in different area affect TC size. The Advanced Research Weather Research and Forecasting (ARW-WRF) model (version 3.5.1) is used in this study to conduct simulations of Typhoon Megi (2016). Several experiments are carried out, including a control (CTL) simulation and sensitivity experiments with surface heat fluxes suppressed in varying degrees and in inner and outer core of TC. The results show that more surface heat fluxes leads to a larger TC. Furthermore, outer core surface heat fluxes are more sensitive to TC size than inner core surface heat fluxes. Suppressing surface heat fluxes weakens the rainbands around the suppressed area. The weakened rainbands slow down the secondary circulation. However, the weakening of secondary circulation resulted from the weakened inner rainbands is constrained in the inner core region, so it only slightly affects the absolute angular momentum import in the outer region. On the other hand, inactive outer rainbands leads to a broader secondary circulation weakening, so the absolute momentum import in the outer region is less, which is not favorable for TC size increase. Therefore, the supplementation of surface heat fluxes in the outer core region is more important than that in the inner core region.